1. Field of the Invention
This invention relates to electric lamps and has particular reference to an improved incandescent lamp of the iodine-cycle type.
2. Description of the Prior Art
Halogen incandescent lamps are per se well known in the art and a lamp of this type in which the regenerative-cycle is established and sustained by a small quantity of elemental iodine (from 0.01 to 1 micromole per cm.sup.3 or ml. of bulb volume) is described in U.S. Pat. No. 2,883,571 issued Apr. 21, 1959 to E. G. Fridrich et al. A method of making such a lamp in which the iodine is introduced into the lamp envelope in the form of an iodide or hydroiodide of an element of Group IV of the Periodic Table (carbon, silicon, titanium, germanium, zirconium, tin, hafnium, lead, and thorium) which is an involatile solid at room temperature is disclosed in U.S. Pat. No. 3,738,729 issued June 12, 1973 to G. E. Coxon et al. CHI.sub.3, CI.sub.4, GeI.sub.4 and SiI.sub.4 are the only compounds specifically disclosed and they are preferred.
The broad concept of dosing an incandescent lamp with a tin halide (such as SnI.sub.2, SnI.sub.4, SnBr.sub.2, SnBr.sub.4, SnCl.sub.2 and combinations thereof) to provide both iodine and a vaporized tin halide in the fill gas is disclosed in Japanese Utility Design Publication No. 1971 -23575 dated Aug. 13, 1971 (applied for by Toshiba Electric Co.). However, no information regarding dosing amounts or ranges for any of the tin halide additives is given. In addition, the lamp is specifically designed to have a bulb wall temperature higher than 500.degree. C in order to surround the filament with vaporized tin halide so that the latter, by virtue of its high molecular weight, will inhibit tungsten evaporation from the filament.
A decorative type incandescent lamp that contains large amounts of elemental iodine (at least 15 milligrams per ml. of envelope volume) either alone or in combination with a vapor of various metallic iodides, including tin iodide, and which is operated in a vertical position and produces a luminous pulsating colored flame effect is disclosed in U.S. Pat. No. 3,384,774 issued May 21, 1968 to J. F. English. The minimum amount of elemental iodine required to produce the pulsating-flame effect is much greater than the maximum amount (about 1 micromole per ml. of envelope volume) required in the conventional iodine-cycle lamps that are disclosed in the aforementioned Fridrich et al patent and are used for general lighting purposes.
A high efficiency incandescent lamp that contains a tungsten filament, a quantity of mercury sufficient to provide from 1 to 20 atmospheres (760 to 15,200 torr) of mercury vapor within the operating lamp, and at least one vaporizable metallic halide (including a tin halide) which is wholly or partially converted to the vapor state and is thus thermally excited to cause the metal to emit light emission of its characteristic atomic spectra is disclosed in U.S. Pat. No. 3,497,754 issued Feb. 24, 1970 to P. D. Johnson.
An incandescent lamp that contains a tantalum carbide filament and an atmosphere (at operating temperature) which comprises hydrogen, vaporized carbon, an inert fill gas, and one or two halogens (at least one of which is iodine or bromine obtained from a metal halide such as a halide of rubidium, cobalt, tin, silver, cadmium, aluminum, copper, zinc, mercury, nickel, and cerium) is disclosed in British patent specification No. 900,200 of H. M. Bryant and D. P. Cooper, Jr. (complete specification published July 4, 1962).
Tungsten-filament lamps that are dosed with other types of halide compounds (such as HgI.sub.2 and HgBr.sub.2) are disclosed in Czechoslovakian Pat. No. 131,567 of J. Pavlata and in British patent specification Nos. 952,938 and 1,105,291 granted to J. Bowtell et al and J. Moore et al, respectively.
Despite the improvements made in halogen-cycle type incandescent lamps, it has been found that when tubular lamps which have been dosed with elemental iodine in the conventional manner and have long singly-coiled filaments and long design lives (over 1,000 hours nominal) are operated in a vertical position, premature blackening of the uppermost portion of the lamp envelope occurs. To avoid this problem, such conventional iodine-cycle lamps are generally restricted to a burning position that is within 4.degree. or so of horizontal. Since this limitation obviously impairs the commercial usefulness of such lamps, a practical and inexpensive way of correcting this deficiency and providing an elongated iodine-cycle lamp which has a singly-coiled filament and can be operated in any position throughout its design life without premature blackening would be a valuable contribution to the art.
Halogen lamps dosed with the usual amount of elemental iodine in accordance with the prior art teachings also have a pale violet-like color or tint which not only distorts the color of the light produced by the lamp but also filters or attenuates the light rays and thus reduces the lamp efficiency.
Hence, an iodine-cycle lamp which would operate with little or no color distortion or filtering of the generated light rays and which has a "universal burning" capability as regards lamp orientation, as well as a long useful life and high efficiency, would be even more desirable and advantageous.
One approach suggested to remedy these shortcomings was to redesign the lamp so that it would have a much higher operating temperature sufficient to cause almost all of the molecular iodine within the lamp to dissociate into iodine atoms (article entitled "Incandescent Bromine Cycle Lamps" by F. A. Mosby et al; April 1967 issue of Illuminating Engineering, p. 198). However, such elevated lamp-operating temperatures are impractical because of the high seal temperatures and other problems which they inherently create.